1. Field of the Invention
This invention relates to a semiconductor device, and in particular to the structure of a device which stabilizes the data writing and clearing properties with a high level of reproducibility of a nonvolatile floating gate memory having a tunnelable insulation film.
2. Description of the Prior Art
A conventional nonvolatile floating gate memory utilizing the tunnel phenomenon customarily has a structure in which a thin SiO.sub.2 film 4 is formed over the entire surface of a channel, as shown in FIG. 1, or over a part within the channel, as shown in FIG. 2, or over an aperture in part of a source-drain diffusion layer as shown in FIG. 3(b), and a floating gate 5 extends over the SiO.sub.2 film 4. FIG. 3(a) is a plane view of FIG. 3(b). A voltage which is effectively positive or negative with respect to the potential of the surface of the Si substrate 1 below the thin SiO.sub.2 film 4 is applied to the gate 5 so that a charge having a different polarity is stored in the floating gate 5, and changes in the threshold voltage of the device are used as data in the memory. In these devices which have a partial tunnelable insulation film as described above, especially in those which have a tunnelable film over an n.sup.+ region 3, however, a sufficient margin must be secured to define the region over which this film is to be formed, otherwise the film area is determined by the masking accuracy. In either of these cases, the area of the tunnelable region is determined by the minimum processing dimensions of the photolithography.
In FIGS. 1 through 3, reference numeral 1 denotes the Si substrate, 2 the source (or drain), 3 the drain (or source), 4 the tunnelable insulation film, 40 the insulation film, 5 the floating gate, and 6 a control gate.
In conventional devices, it is necessary that the tunnelable region is electrically connected to the source-drain region or to the channel region. For this reason, the tunnelable region is generally formed over the source or drain or within the channel.
3. List of the Prior Art
The following is cited as an example of the state of art:
Japanese Patent Laid-Open No. 9388/1975